1. Field of the Invention
The present invention generally relates to the field of surface analyzers and more particularly to transparent wafer or disk inspection.
2. Description of Background Art
A problem in the inspection of transparent wafers is the detection of top surface defects without detecting defects from the bottom side of the transparent wafers. Conventional methods for detecting particles (defects) within or upon the top surface of a transparent wafer rely upon illuminating a sample with a beam of light that has been channeled through a modest numerical aperture lens. In these systems, the incident light expands once it leaves the focal point and the intensity of the light decreases as the beam passes through the transparent wafer. The incoming light produces a significant scattering signal when interacting with surface particles and near surface defects on the transparent wafer, while the scattering signal from sub-surface and back-surface defects is substantially reduced by the divergence of the incoming beam (reduction in beam intensity). These types of methods are limited to investigation of relatively thick wafers (on the order of 2.0 mm), and the divergence of the beam does not eliminate the signal from the back surface it only reduces it. When analyzing thin transparent wafers with a modest numerical aperture system, the beam which passes through the wafer has sufficient intensity that it becomes difficult to distinguish whether a defect is on the top or bottom of a transparent wafer. Other conventional defect detection systems provide a wavelength of light for illumination that is selected to match an area where the transparent wafer appears opaque, thus minimizing the influence of light scattering from the subsurface or back side of the transparent material. However, many defects are left undetected due to a lack of penetration depth from the incident light. Another problem occurs when transparent wafers have only their top surface polished. The beam which penetrates through the transparent wafer and strikes the bottom (unpolished) surface generates a large scattered light signal. This large scattered light signal from the bottom surface obscures the scattered light from defects on the top surface making top surface defect detection difficult or impossible.
What is needed is a system and method that (1) analyzes transparent wafers using a single sided system, (2) distinguishes the position of scattering sites, (3) measures the effects of scattering only from scattering sites located within an defined depth of the top surface of the wafer, and (4) permits modification of the depth of the scattering sites that are measured.